I would guess tou would have to render the animation as a series of stills (Poser allows that) in a format that supports alpa channels, like TIF, then use something like Adobe Premier to put them togeather. But it might be easier to just have a blue or green background (you want it as far from the colors you use as possable) and use something like Premier's hue overlay (I forget what it is called, but I've done it) to make it transparent.